In memory applications in the semiconductor industries, such as DRAM and 2D NAND, plasma etching removes silicon-containing layers, such as SiO or SiN layers, from semiconductor substrates. For novel memory applications, such as 3D NAND (US 2011/0180941 to Hwang et al.), high aspect ratio etching of stacks of multiple SiO/SiN or SiO/poly-Si layers is critical. Preferably, the etchant has high selectivity between the mask and layers being etched. Furthermore, the etchant preferably etches the structure such that the vertical profile is straight with no bowing. The 3D NAND stack may include other silicon containing layers.
Traditionally, plasma etching is carried out using a plasma source which generates active species from a gas source (such as hydrogen-, oxygen-, or fluorine-containing gases). The active species then react with the Si-containing layers to form a fluorocarbon blocking overlayer and volatile species. The volatile species are removed by low pressure in the reactor, which is maintained by a vacuum pump. Preferably, the mask material is not etched by the active species. The mask material may comprise one of the following: photoresist, amorphous carbon, polysilicon, metals, or other hard masks that do not etch.
Traditional etch gases include cC4F8 (Octafluorocyclobutane), C4F6 (Hexafluoro-1,3-butadiene), CF4, CH2F2, CH3F, and/or CHF3. These etch gases may also form polymers during etching. The polymer acts as protection layers on the sidewalls of the pattern etch structure. This polymer protection layer prevents the ions and radicals from etching the sidewalls which could cause non-vertical structures, bowing, and change of dimensions. A link between F:C ratio, SiO:SiN selectivity, and polymer deposition rate has been established (see, e.g., Lieberman and Lichtenberg, Principles of Plasma Discharges and Materials Processing, Second Edition, Wiley-Interscience, A John Wiley & Sons Publication, 2005, pp. 595-596; and FIG. 5 of U.S. Pat. No. 6,387,287 to Hung et al. showing an increased blanket selectivity to nitride for lower values of the F/C ratio).
Traditional dry etch methods, such as chemical etching, may not provide the necessary high aspect ratio (>20:1) because the high pressure conditions required during chemical etching may have detrimental effects on the aperture formed. Traditional chemistries, such as C4F8 and C4F6, may also be insufficient to provide the high aspect ratio required because the etch manufacturers are rapidly depleting the available parameters used to make the traditional chemistries work, such as RF power, RF frequency, pulsing schemes and tuning schemes. The traditional chemistries no longer provide sufficient polymer deposition on high aspect ratio side walls during the plasma etching process. Additionally, CxFy, wherein x and y each independently range from 1-4, polymers on sidewalls are susceptible to etching. As a result, the etched patterns may not be vertical and structures may show bowing, change in dimensions, and/or pattern collapse.
One key issue with etching of patterns is bowing. Bowing is often due to sidewall etching of the mask layer, which is often an amorphous carbon material. Amorphous carbon materials can be etch by oxygen radicals in the plasma which can cause increased opening of the mask and result in a bow-like etch structure.
U.S. Pat. No. 6,569,774 to Trapp discloses a plasma etch process for forming a high aspect ratio contact opening through a silicon oxide layer. Trapp discloses inclusion of nitrogen-comprising gases such as NH3 to fluorocarbon (CxFy) and fluorohydrocarbon (CxFyHz) etch chemistries to improve resist selectivity and reduce striations. A list of 35 fluorocarbon and fluorohydrocarbon chemistries are disclosed, but no structural formulae, CAS numbers, or isomer information are provided.
WO2010/100254 to Solvay Fluor GmbH discloses use of certain hydrofluoroalkenes for a variety of processes, including as an etching gas for semiconductor etching or chamber cleaning. The hydrofluoroalkenes may include a mixture of at least one compound selected from each of the following groups a) and b):                a) (Z)-1,1,1,3-tetrafluorobut-2-ene, (E)-1,1,1,3-tetrafluorobut-2-ene, or 2,4,4,4-tetrafluorobut-1-ene, and        b) 1,1,1,4,4,4-hexafluorobut-2-ene, 1,1,2,3,4,4-hexafluorobut-2-ene, 1,1,1,3,4,4-hexafluorobut-2-ene and 1,1,1,2,4,4-hexafluorobut-2-ene.        
State of the art vertical 3D NAND structures require very high aspect ratios through alternating stacks of materials.
A need remains for new etch gas compositions for use in plasma applications to form high aspect ratio apertures.